Within the field scanning probe microscopy (SPM), such as atomic force microscopy (AFM), particular attention is paid to proper sensing and mapping of high aspect ratio features. High aspect ratio structures are structures on the surface of the substrate that comprise one or more sidewalls having an angle (relative to the normal on the surface) that is smaller than half the cone angle of the probe tip for e.g. a symmetric cone shaped or triangular prism shaped tip. For these high aspect ratio structures, the sidewalls are thus steeper than the angle of the probe tip. Therefore, the angle of the probe tip in these cases prevent accurate determination of the shape of the high aspect structure.
The problem is exemplarily illustrated in FIG. 1A to 1C. In FIG. 1A, a substrate 1 comprises a substrate surface 3 that is to be scanned using a scanning probe microscopy device in order to map the structures present on the surface 3. The surface 3 comprises a nanostructure 5. Nanostructure 5 comprises side walls 6-1 and 6-2 forming straight angles with the plane of the surface 3, i.e. being parallel to the normal through the surface 3. Thus, the angle between the surface 3 and any of the walls 6-1 or 6-2 is approximately π/2 radians. The angle between any of walls 6-1 and 6-2 and the normal to the surface is 0 radians.
In FIG. 1B, a probe tip 10 of the scanning probe device is schematically illustrated in cross section. The probe tip 10 is cone shaped, and the angle α is the angle between the opposing sides 11 and 12 of the probe tip which defines the sharpness of the cone. The axis 4 defines the axis of symmetry of the probe tip 10. If the surface would be flat or if it would only comprise shallow structures with gradual edges or side faces, there would be no trouble for the probe tip 10 to follow the surface perfectly accurate. In the situation of FIGS. 1A to 1C, the nanostructure 5 is a high aspect ratio nanostructure having very steep walls 6-1 and 6-2 relative to the surface 3. Defined differently, the angle between the normal through the surface 3 and any of the walls 6-1 or 6-2 of the nanostructure 5 is smaller than half of the angle α of the probe tip. In this situation, during scanning of the probe tip 10 across the surface 3, the probe tip follows the scan path 15. This scan path 15 is smoothed at an exaggerated amount upon encountering the walls 6-1 and 6-2. This is because the angled sides 11 and 12 experience van der Waals forces at the edges of the walls 6-1 and 6-2 of nanostructure 5. Smoothening is due to limited tip sharpness, and limited tip-sample control bandwidth.
The result of scanning of the high aspect ratio nanostructure 5 is illustrated in FIG. 1C which shows the resulting sensor signal 18 from the scan of nanostructure 5. The scanning probe microscopy device is not able to accurate map the walls 6-1 and 6-2 and the edges of the nanostructure 5.
A known solution to overcome the above problem is illustrated in FIGS. 2A to 2C. FIG. 2A again shows the nanostructure 5 on the surface 3 of the substrate 1, having high aspect ratio side walls 6-1 and 6-2. To measure and map the walls 6-1 and 6-2 more accurately, scanning is now performed in two directions with a tilted probe tip 10, as illustrated in FIGS. 2B and 2C. In FIG. 2B, the probe tip 10 is tilted with it's symmetry axis backward with respect to the scanning direction (which is in FIG. 2B from left to right). Due to the tilting, side 12 of probe tip 10 no longer limits the accuracy of the measurement upon encountering the wall 6-1. However, this gained accuracy for wall 6-1 comes at the cost of accuracy for sensing wall 6-2, because going from left to right with probe tip 10, the side 11 of the probe tip will limit accuracy of mapping even more at wall 6-2. This therefore results in scan path 20.
To resolve the inaccuracy at wall 6-2, the same scan is performed in the reverse direction by moving the probe tip 10 from right to left. This is illustrated in FIG. 2C. During scanning in the other direction, also the tilting of the probe tip is reversed. Probe tip 10 now follows scan path 21, which accurately maps wall 6-2 at the cost of accuracy at wall 6-1. The both paths 20 and 21 are combined, and the resulting sensor signal 23 providing the mapping of nanostructure 5 is illustrated in FIG. 2D. As can be seen, the walls 6-1 and 6-2 have been mapped at a much better accuracy as compared to the mapping result illustrated in FIG. 1C.
Although the above method works well for obtaining an accurate mapping of the structures, the measuring method has several drawbacks. For example, the probe must be tilted and the tilt angle of the probe relative to the normal through the surface 3 must be exactly the same but mirrored in the respective scanning directions. The accuracy of the tilting is difficult to achieve. Moreover, the tilting requires a hinge structure and an additional tilting actuator in the metrology loop of the system. These elements introduce additional inaccuracy by comprising parts that are differently sensitive to temperature variations, and by having increased sensitivity to external vibrations. Moreover, to prevent crashing of the probe into the sample surface, dedicated SPM probes are required being suitably shaped.